News reports say members of the Islamic State group have started advertising women and children for sale on social networking applications such as WhatsApp, Facebook and Telegram.

The Associated Press is reporting that IS is advertising via encrypted messages the "sale" of Yazidi women, an ethnic group indigenous to Iraq that has been targeted by IS.

A spokesman for the U.S. State Department, John Kirby, told AP the United States continues "to be appalled by credible reports that Daesh [Islamic State] is trafficking in human beings, and sex slavery in particular."

"This depravity," he said, "not only speaks to the degree to which Daesh cheapens life and repudiates the Islamic faith, it also strengthens our resolve to defeat them."

Islamic State kidnapped thousands of Yazidi women and children in August 2014 when it attacked their homes and villages in northwestern Iraq near the border with Syria. Iraqi Kurdish forces backed by U.S. airstrikes have since recaptured some of that territory, but many children have been orphaned and many young women are believed to have been taken into slavery.

The slaves are said to be held against their will and often subjected to rape as well as forced labor. While some women have escaped, they say the slaves are closely tracked and are often recovered, only to face punishment.

One escapee, Nadia Mourad, has spoken to U.S. and European lawmakers about getting more help for the Yazidis.

"They are being used as human shields," she told the European Parliament. "They are not allowed to escape or flee. Probably they will be assassinated. Where is the world in all this? Where is humanity?"

A representative for Telegram, a messaging app, said the company regularly removes public channels used by Islamic State and is committed to preventing abuse of the service.